1. Field of the Invention
The present invention relates to a method of checking the bonding of a cellular structure, such as those termed "honeycomb structures" to a support, and also to apparatus for carrying out the method.
In this specification the term "honeycomb" is intended to refer to any kind of cellular structure, whatever may be the geometry of the cells. Similarly, it is to be understood that the term "bonding" does not imply any restrictions as to the method of achieving the bonding. The invention is applicable to the checking of any method of binding the honeycomb to its support, whether by glueing, welding, brazing or any other method of binding.
More and more the aeronautical industry is making use of honeycomb structures, because of their lightness and resistance to crushing, for structural reinforcement purposes, for example in turbomachine casings and aircraft floors. Another example is the fan casing of a turbojet engine, outside which it is possible to arrange an annular strip of honeycomb reinforcement in line with the fan blades to replace the reinforcing ribs which have been used previously and the machining of which required a substantial extra thickness of the initial component in relation to the finished component.
The honeycomb structure thus used is a particularly effective reinforcement structure provided it is bonded to the support at the bottom of all of its cells. To make sure of this, it is necessary to carry out a very tight statistical verification of this bonding.
2. Summary of the Prior Art
At present, this examination is made by endoscopic checking of the bottoms of the cells by very powerful illumination of a cell and examining the bottom of the adjacent cell to see whether a bonding fault allows a ray of light to pass between the illuminated cell and the adjacent cell.
This check is carried out statistically by inspecting the strip of honeycomb at three or four points across its width every five centimeters along its length. Thus, the checking of a 15 cm wide strip of honeycomb bonded as a reinforcement on a fan casing of 183 cm diameter requires a total checking time which may exceed eight hours, resulting in substantial visual fatigue for the operator and a checking reliability which decreases with time.
The examination may also be carried out by capillarity using a fluorescent liquid. This makes it necessary to use very wetting liquids, which often comprise constituents dangerous to the operator. Moreover, checking a casing requires immersing it in a vat to wet it, thus requiring bulky installations, and the checking operation itself takes as long as that using endoscopic verification.